1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate, for example, a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus transfers a circuit pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system.
A minimum dimension (resolution) with which the projection exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of a projection optical system. Along with the recent demand for micropatterning semiconductor devices, the wavelength of the exposure light is shortening and the NA of the projection optical system is increasing. For example, to shorten the wavelength of the exposure light, a KrF excimer laser (wavelength: about 248 nm) has conventionally been used as the exposure light. In recent years, however, an ArF excimer (wavelength: about 193 nm) is used as the exposure light. To increase the NA of the projection optical system, a projection optical system having a numerical aperture more than 0.9 has been developed. In recent years, however, a projection optical system (immersion projection optical system) which uses an immersion exposure technique and has a numerical aperture more than 1.0 has been proposed. The immersion exposure technique further increases the NA of the projection optical system by filling the space between the wafer and a final lens (final surface) of the projection optical system with a liquid.
The immersion projection optical system generally uses pure water as a liquid which fills the space between the final lens and the wafer, and uses quartz as the glass material of the final lens. From the viewpoint of the arrangement of this system, the limit value of the numerical aperture is about 1.35. Under these circumstances, it has been proposed to increase the numerical aperture to 1.5 or 1.65 or more by using a liquid having a refractive index higher than that of pure water and a glass material having a refractive index higher than that of quartz.
At present, LuAG (Lu3Al5O12) is receiving a great deal of attention as a glass material which transmits light having a wavelength of 193 nm and has a refractive index higher than that of quartz. Since, however, LuAG is a crystal glass material, it shows birefringence attributed to its crystal structure. As the refractive index of LuAG increases, the birefringence of LuAG attributed to its crystal structure also increases. For example, CaF2 (calcium fluoride) has a refractive index of 1.506 with respect to light having a wavelength of 193 nm, and has a maximum birefringence of 3.4 nm/cm attributed to its crystal structure. On the other hand, LuAG has a refractive index of 2.14 with respect to light having a wavelength of 193 nm, and has a maximum birefringence of 30 nm/cm attributed to its crystal structure.
When the immersion projection optical system has a numerical aperture more than 1.0, the surface (the surface directly above the wafer) of the final lens on the wafer side is generally made flat to stably control the liquid between the final lens and the wafer. When a numerical aperture NA of the projection optical system and a refractive index nFL of the final lens are determined, a maximum angle θMX between the optical axis of the projection optical system and a light beam passing through the final lens is defined by:θMX>arcsin(NA/nFL)×180/Π[°]  (1)
FIG. 13 shows the dependence of an angle θFL between the optical axis of the projection optical system and a light beam passing through the final lens upon the numerical aperture when the glass material of the final lens is LuAG (refractive index: 2.14). In FIG. 13, the ordinate indicates the angle θFL, and the abscissa indicates the numerical aperture of the projection optical system. Referring to FIG. 13, the angle θFL is 44.5° when the numerical aperture is 1.5, while it is 50° when the numerical aperture is 1.65.
FIGS. 14A and 14B each show the birefringence distribution of an isotropic crystal glass material (flat shape) attributed to its crystal structure. FIG. 14A shows a birefringence distribution around the <1 1 1> crystal axis (crystal orientation). FIG. 14B shows a birefringence distribution around the <1 0 0> crystal axis (crystal orientation). In FIGS. 14A and 14B, each position along the radial direction indicates the passage angle of a light beam, and each position along the azimuth direction indicates the passage orientation angle of the light beam. The length of each short line indicates the relative birefringence amount, and the direction of the short line indicates the fast axis orientation of birefringence.
Referring to FIGS. 14A and 14B, the birefringence of the isotropic crystal glass material attributed to its crystal structure is zero in the <1 0 0> crystal axis orientation and <1 1 1> crystal axis orientation, and it takes a maximum value in the <1 1 0> crystal axis orientation. Hence, when the <1 0 0> crystal axes and <1 1 1> crystal axes are oriented along the optical axis of the projection optical system, the passage angle of the light beam increases as the numerical aperture increases, resulting in an increase in birefringence attributed to the crystal structure.
To correct the birefringence attributed to the crystal structure, there has been proposed a technique of forming other lenses of the projection optical system using the same crystal glass material as used for the final lens or a crystal glass material having nearly the same birefringence as that used for the final lens, and controlling the assembly angle of the crystal glass material around the optical axis. There has conventionally been proposed another technique of correcting the birefringence attributed to the crystal structure. Japanese Patent Laid-Open Nos. 2004-45692 and 2006-113533 can be referred to as these techniques.
Unfortunately, since LuAG used for the final lens of the projection optical system is very expensive, it is desirable to avoid using it for other lenses of the projection optical system as much as possible. Furthermore, since LuAG has a low transmittance (has high light absorption) and its refractive index greatly changes in response to a change in temperature, it is also desirable to avoid using LuAG as much as possible to suppress aberration fluctuation in exposure.
Japanese Patent Laid-Open No. 2004-45692 discloses a technique of efficiently correcting the birefringence attributed to the crystal structure by orienting the <1 0 0> crystal axes of a crystal glass material exhibiting a maximum angle of 30° or more between a passing light beam and the optical axis of the projection optical system along the optical axis of the projection optical system. However, Japanese Patent Laid-Open No. 2004-45692 does not take account of a high refractive index material which has a very large birefringence (e.g., has a birefringence more than 20 nm/cm) attributed to its crystal structure. It is difficult to correct the birefringence of such a high refractive index material attributed to its crystal structure unless not only the condition of a crystal glass material in which the <1 0 0> crystal axes are oriented along the optical axis of the projection optical system but also the condition of a crystal glass material in which the <1 1 1> crystal axes are oriented along the optical axis of the projection optical system is defined.
Japanese Patent Laid-Open No. 2006-113533 discloses a technique of correcting the birefringence of a high refractive index crystal glass material attributed to its crystal structure by forming the final lens and lenses adjacent to it using MgO (magnesium oxide) and CaO (calcium oxide) having birefringences of opposite signs attributed to their crystal structures. However, Japanese Patent Laid-Open No. 2006-113533 does not define concrete arrangements of the crystal axes of MgO and CaO, which allow reduction in birefringence attributed to the crystal structure. In practice, high-quality MgO and CaO which can be used for the exposure apparatus neither exist nor are under development.